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down Björn Wallace, David Cesarini, Paul Lichtenstein, and Magnus Johannesson
Heritability of ultimatum game responder behavior
PNAS 104: 15631-15634; published online as 10.1073/pnas.0706642104

Abstract 1 of 1backSOCIAL SCIENCES / BIOLOGICAL SCIENCES / ECONOMIC SCIENCES / ANTHROPOLOGY
Heritability of ultimatum game responder behavior

Björn Wallace*, David Cesarini{dagger},{ddagger}, Paul Lichtenstein§, and Magnus Johannesson*

*Department of Economics, Stockholm School of Economics, Box 6501, SE-113 83 Stockholm, Sweden; {dagger}Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142; and §Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77 Stockholm, Sweden

Edited by Henry C. Harpending, University of Utah, Salt Lake City, UT, and approved August 24, 2007 (received for review July 16, 2007)

Experimental evidence suggests that many people are willingto deviate from materially maximizing strategies to punish unfairbehavior. Even though little is known about the origins of suchfairness preferences, it has been suggested that they have deepevolutionary roots and that they are crucial for maintainingand understanding cooperation among non-kin. Here we reportthe results of an ultimatum game, played for real monetary stakes,using twins recruited from the population-based Swedish TwinRegistry as our subject pool. Employing standard structuralequation modeling techniques, we estimate that >40% of thevariation in subjects' rejection behavior is explained by additivegenetic effects. Our estimates also suggest a very modest rolefor common environment as a source of phenotypic variation.Based on these findings, we argue that any attempt to explainobserved ultimatum bargaining game behavior that ignores thisgenetic influence is incomplete.

cooperation | experimental economics

Author contributions: B.W., D.C., P.L., and M.J. designed research;B.W., D.C., and M.J. performed research; B.W., D.C., P.L., andM.J. analyzed data; and B.W., D.C., P.L., and M.J. wrote thepaper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

There is too little variation in the proposal stage to estimatemodel parameters with reasonable precision, and hence we focuson responder behavior. The equality of distributions of acceptancethresholds by zygosity was tested with a design-based independencetest, which takes the correlation between twins into account.The {chi}2 statistic is adjusted by using the second-order correctionof Rao and Scott (11).

|| To estimate the mean acceptance threshold, the acceptance thresholdis set at the middle value of the acceptance threshold intervals.

** The maximum likelihood estimation is implemented in Mx, a numericaloptimizer for behavior genetics (12). We estimate a thresholdmodel based on the response categories for the acceptance thresholdin the experiment. The programming code for the threshold modelwe estimate is adapted from the digital scripts library at theUniversity of Amsterdam (13). The algorithm that estimates confidenceintervals for the parameters is explained in some detail byNeale and Miller (14).

{dagger}{dagger} One interpretation, suggested to us by a referee, is that thehigh heritability casts doubt on accounts of ultimatum rejectionsrelying on notions of evolutionary disequilibrium. In lightof our findings, it seems plausible that the trait has beenunder stabilizing selection in the post-Pleistocene era.

{ddagger}{ddagger} Previous experimental research has shown that it is fairly commonto observe some subjects rejecting both low (<50%) and high(>50%) offers (37). In the parlance of experimental economics,these subjects are "hyperfair." In our data, slightly fewerthan one-third of the subjects exhibit such behavior.

§§ For example, a response in which an individual accepts an offerof 10% but then rejects an offer of 20% is considered inconsistentbecause an acceptance threshold is not uniquely defined on theinterval from 0 to 50%.

¶¶ Furthermore, our results are robust to the manner in which wedeal with these inconsistent responses. Dropping the five inconsistentresponses clarified by e-mail has no discernible effect on theheritability estimates or their significance levels.

{ddagger}To whom correspondence should be addressed. E-mail: cesarini{at}mit.edu

© 2007 by The National Academy of Sciences of the USA

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